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Infect Immun, July 1998, p. 3443-3446, Vol. 66, No. 7
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Susceptibility of Epithelial Cells to Pseudomonas
aeruginosa Invasion and Cytotoxicity Is Upregulated by Hepatocyte
Growth Factor
Suzanne M. J.
Fleiszig,1,*
Vicky
Vallas,1
Cindy H.
Jun,1
Leo
Mok,1
Daniel F.
Balkovetz,2,
Michael G.
Roth,3 and
Keith E.
Mostov2
The Morton D. Sarver Laboratory for Cornea
and Contact Lens Research, School of Optometry, University of
California, Berkeley,1 and
Department of
Anatomy and Biochemistry, University of California, San
Francisco,2 California, and
University
of Texas Southwestern Medical Center at Dallas, Dallas,
Texas3
Received 6 March 1998/Accepted 16 April 1998
 |
ABSTRACT |
Normal cell polarity protects epithelial cells against
Pseudomonas aeruginosa invasion and cytotoxicity. Using
epithelial cell clones with selective defects in sorting of membrane
constituents, and using hepatocyte growth factor pretreatment, we found
that polarized susceptibility to P. aeruginosa can be
altered without disrupting tight junctions. The results also showed
that cellular susceptibility factors for invasion and cytotoxicity are
not the same, although both are localized to the basolateral cell
surface in polarized epithelial cells.
 |
TEXT |
Pseudomonas aeruginosa
clinical isolates can be broadly classified into two types, those that
invade epithelial cells (invasive) (6, 7) and others that
are cytotoxic toward epithelial cells (1, 8). These two
phenotypes are associated with genetic differences in the
ExsA-regulated pathway of genes on the bacterial chromosome
(9). Healthy tissues are normally resistant to infection with both types of P. aeruginosa isolates (23).
Epithelial cells throughout the body are polarized; that is, they have
distinct apical and basolateral membranes that are separated by tight
junctions. In intact healthy tissues, only the apical membrane is
exposed to the environment. Using methods that disrupt the integrity of
tight junctions, we have shown that the basolateral membranes of
epithelial cells are more susceptible to both P. aeruginosa
invasion and cytotoxicity than is the apical cell membrane
(10).
Injury can expose basolateral cell surfaces, and this may, in part,
explain why tissues become susceptible to infection with P. aeruginosa after overt injury (23). However, P. aeruginosa infection can occur without overt injury; e.g., during
contact lens wear, in cystic fibrosis, and in immunocompromised
patients.
Polarity of epithelial cells is determined by sorting of membrane
constituents to, from, and between apical and basolateral cell membrane
compartments. Tight junctions function to maintain polarity subsequent
to sorting (12). The aim of this study was to determine
whether or not the susceptibility of apically exposed cell membranes to
P. aeruginosa invasion and cytotoxicity could be altered
without disrupting tight junctions between epithelial cells.
Hepatocyte growth factor (HGF) is a polypeptide growth factor with
multiple functions on epithelial cells and tissues, including mitogenesis, cell motility, and the development and regeneration of
organs (13, 14, 16, 18). Recently, HGF has also been found
to alter normal polarized sorting of apical and basolateral membrane
components in Madin-Darby canine kidney II (MDCK) cells without
disrupting tight junctions when applied to the basolateral cell
membrane (2).
Effect of HGF pretreatment.
The basolateral surface of
filter-grown MDCK cells was pretreated with HGF by adding 0.1-µg/ml
HGF to the basolateral growth medium and incubating the cells for
72 h. Susceptibility of HGF-treated and control MDCK cells to
P. aeruginosa 6294 adherence and invasion and to the
cytotoxicity of strain 6206 was measured.
MDCK cells were grown as previously described (10).
Transepithelial resistance (TER) of cells grown on filters was
monitored by using an EVOM meter (World Precision Instruments Inc.,
Sarasota, Fla.). TER was measured to ensure that tight junctions were
intact and that cells were morphologically polarized. The baseline TER, determined by measuring resistance across filters treated in the same
manner without cells, was approximately 100
/cm2;
TER increased when cells of all types became confluent, indicating that
tight junctions had formed. Bacterial viable counts, gentamicin survival assays, and trypan blue exclusion assays were used to measure bacterial adherence, invasion, and cytotoxic activity as
previously described (8, 10). The t test was used
for all statistical analyses.
The results of HGF treatment mirrored the effect of disrupting
tight junctions; there was enhanced susceptibility of MDCK
cells
to adherence (
P = 0.003), invasion (
P = 0.0001), and cytotoxicity
by
P. aeruginosa (Table
1 and Fig.
1). The TER of the cells was
not reduced
by HGF; in fact, the treatment increased TER slightly
(
P = 0.04), confirming that HGF treatment did not
disrupt tight
junctions (Table
1).

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FIG. 1.
Effect of HGF (0.1 µg/ml) pretreatment on cell
susceptibility to P. aeruginosa cytotoxicity. A, HGF
pretreatment; B, no HGF pretreatment; C, HGF pretreatment with no
bacterial exposure.
|
|
Role of E-cadherin in HGF-induced invasion.
The role of
E-cadherin in HGF-induced bacterial invasion was explored, since it has
been found to be a receptor for basolateral internalization of
Listeria monocytogenes (20) and because it is
sent to the apical surface of HGF-treated MDCK cells (2). The apically exposed surface of HGF-treated MDCK epithelial cells was
preincubated with DECMA-1 (anti-uvomorulin [E-cadherin] rat immunoglobulin G1 in ascites; Sigma) for 30 min prior to addition of
bacteria in the presence of the antibody and incubation for 3 h.
Control HGF-treated cells were incubated with rat ascites immunoglobulin G against ZO1 (Chemicon International) instead of
DECMA-1. Antibodies against E-cadherin (DECMA-1) were found to inhibit
cell invasion by L. monocytogenes 10403S but not by P. aeruginosa 6294 after HGF pretreatment (data not shown).
Similarly, antibodies to E-cadherin did not block P. aeruginosa 6206 cytotoxicity (data not shown).
Effect of sorting defects.
Since HGF might induce effects on
cells that alter susceptibility to bacteria other than the apical
exposure of basolateral membrane factors, bacterial interactions with
two MDCK cell clones with different sorting defects were explored.
MDCK II/J cells are likely to have a defect in sphingolipid synthesis
and missort Na/K ATPase but not E-cadherin (
19).
MDCK clone C13 cells were obtained after transfection of MDCK cells
obtained from the American Type Culture Collection with
a plasmid
expressing a mutant influenza virus hemagglutinin that
contains a
basolateral sorting signal (
3). C13 cells have a
selective
sorting defect (
4) that appears to be quite different
from
the defect in the clone II/J cells described above. After
infection
with influenza virus, the hemagglutinin protein is sent
to the apical
membrane as in control C15 cells; however, transferrin
and methionine
receptors are not restricted to the basolateral
surface. Vesicular
stomatitis virus infects exclusively through
the basolateral
surface of C15 cells but preferentially from the
apical surface of C13
cells. For C15 cells, 80% of an endogenously
secretory glycoprotein,
gp80, is secreted apically, but for C13
cells, apical and basolateral
cell membranes secrete equal amounts
of gp80.
Cells from each of these lines were found to produce TERs across
monolayers that were at least as high as those of their respective
control clone cells (Table
2). C13 clone
cells produced TERs
similar to those of C15 control clone cells
(
P = 0.6), and II/J
cells produced a higher TER than
their control II/G cells (
P =
0.0001). These results
showed that all of the cell types studied
were able to form functional
tight junctions when grown as described.
Both of the sorting-defective cell types (C13 and clone II/J cells)
were more susceptible to the cytotoxicity of
P. aeruginosa 6206 than were their respective control clone cells (C15 and clone
II/G) (Fig.
2), providing further
evidence that factors involved
in susceptibility to
P. aeruginosa cytotoxicity are associated
with the basolateral
membrane in polarized epithelial cells.

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FIG. 2.
Susceptibility of cells with partial sorting defects to
P. aeruginosa cytotoxicity. A, sorting-defective clone II/J
cells; B, clone II/G cells (control for clone II/J cells); C,
sorting-defective C13 cells; D, C15 cells (control for C13 cells).
Clear areas in the centers of the foci are areas where cells have been
completely destroyed.
|
|
The effects of these sorting defects on invasion by invasive strain
6294 were entirely different from their effects on cytotoxicity.
C13
cells were equally susceptible to
P. aeruginosa invasion as
the control C15 cells (
P = 0.16), suggesting that
factors involved
in invasion were not missorted by the C13 cells. Clone
II/J cells
were even less susceptible to
P. aeruginosa
invasion than were
the control clone II/G cells (
P = 0.003) (Table
2). The most
likely explanation for this result is that
there may also be apical
factors on correctly polarized MDCK cells that
are involved in
invasion; i.e., some apical receptors for invasion may
be sent
basolaterally in II/J cells (based on the assumption that, in
fact, II/G cells sort all membrane constituents "correctly").
In
this regard, CFTR has recently been shown to be a receptor
for
P. aeruginosa invasion of certain epithelial cell types
(
22);
when present, CFTR is usually an apically expressed
protein. The
low-level presence of apical receptors on MDCK cells might
explain
why
P. aeruginosa can invade confluent heterogeneous
MDCK cells,
albeit to a lesser extent than MDCK cells with disrupted
tight
junctions. This is clearly not the case for some epithelial cell
types that appear to be more efficiently polarized for susceptibility;
i.e., the apical surfaces of human nasal epithelia do not permit
P. aeruginosa invasion at all, while their basolateral
surfaces
are susceptible (
10).
When cytotoxic
P. aeruginosa kills MDCK or corneal
epithelial cells, it begins by killing only occasional single cells.
Once
the first cell is killed, bacteria gain access to the basolateral
sides of adjacent cells in the monolayer and spread outwards to
form
foci of dead cells (
10). This might be explained by some
form of random activity that is related to the bacteria, since
cytotoxicity is regulated in cytotoxic
P. aeruginosa by ExsA
and
is contact dependent (
11,
25). Alternatively, the
pattern
might reflect heterogeneity in the efficiency of polarity among
a population of cells growing as a monolayer. In this regard,
the
"sorting-defective" C13 and II/J clone cells that were used
in this
study were originally isolated from heterogeneous populations
of MDCK
cells. Interestingly, cytotoxicity still occurred as isolated
foci with
the C13 and II/J cell clones, although there were many
more foci. In
this case, differences in cell maturity or development
of individual
cells might account for the pattern of killing.
These results, which showed that susceptibility factors for invasion
and cytotoxicity are not the same, support the conclusions
of other
studies in this laboratory that
P. aeruginosa-induced
cytotoxicity occurs from outside of the target epithelial cell
(
5). Whether this indicates that different receptors are
involved
has yet to be determined.
The effect of HGF on cell polarity could help explain why tissue injury
and inflammation are risk factors for
P. aeruginosa infection. HGF production increases during various pathological
states
of various organs with epithelial cell layers, such as
the eye, kidney,
liver, lung, and pancreas (
15,
17,
18,
21,
24,
26). The
results presented here suggest the potential
for HGF, and therefore
possibly other factors, to alter susceptibility
to bacterial virulence
factors in the absence of direct injury
to the affected cells. They
also raise the possibility that injury
at one site could lead to
enhanced susceptibility of cells distant
from the site of the injurious
stimuli.
 |
ACKNOWLEDGMENTS |
This work was supported by NIH grant RO1 EY11221 and a UC Berkeley
Faculty Research Grant to S.M.J.F.; by grant RO1 HL55980 to K.E.M., who
is an established investigator of the American Heart Association; and
by grant RO1 GM3754 to M.G.R.
We thank James Nelson of Stanford University for the II/J and II/G MDCK
cell clones and Ralph Schwall at Genentech for generous gifts of
recombinant human hepatocyte growth factor.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: School of
Optometry, University of California, Berkeley, CA 94720-2020. Phone:
(510) 643-0990. Fax: (510) 643-5109. E-mail:
fleiszig{at}socrates.berkeley.edu.
Present address: Departments of Medicine and Cell Biology,
University of Alabama at Birmingham and Department of Veterans Affairs,
Birmingham Veterans Affairs Medical Center, Birmingham, Alabama.
Editor: P. J. Sansonetti
 |
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Infect Immun, July 1998, p. 3443-3446, Vol. 66, No. 7
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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